The present invention relates to chain tensioners and more particularly to an improved hydraulic dashpot chain tensioner for the valve timing chain of an internal combustion engine.
Heretofore, certain internal combustion engines for motor vehicles, notably, the engines for the high performance sports automobiles, employ metallic roller-type timing chains for driving the camshaft in synchronism with the crankshaft of the engine. To assure trouble-free operation of these engines, it is particularly important to apply constant tension to the timing chains so as to eliminate any vibration or snaking of the chains over the full range of engine operating speeds. The problem of providing constant tension of such timing chains has long been recognized by the prior art and a number of devices have been proposed to solve this problem. These prior art devices have met with reasonable success in providing a means for maintaining constant tension on the timing chains, however, there are other difficulties with the prior art devices associated with the operating conditions of the automobile which result in occasional failure of the chain tensioner. Although failures of the known devices are infrequent, when they occur, the result can be catastropic. For example, when one known type of chain tensioner fails, it can happen that the timing chain becomes sufficiently slack that it jumps from the driving sprocket wheels and drives the camshaft so far out of synchronism with the crankshaft as to cause a valve to be driven into a piston head of the engine. Other failures have less catastrophic effects but can result in breakage of or damage to the timing chain requiring costly repairs.
Typically, the prior art chain tensioners include a housing fixedly mounted within the timing chain cover on the engine block. A cylinder, open at one end, is formed in the chain tensioner housing for receiving a piston having a plunger arranged to bear against a pivotally mounted support arm for a sprocket wheel to urge the sprocket wheel against the chain and keep the latter in tension. The piston is biased outwardly by a coil spring and the closed end of the cylinder behind the piston is filled with lubricating oil to form a dashpot for absorbing large, short duration loads, such as might be experienced by whipping or snaking of the timing chain, and for maintaining constant tension on the chain. The chain tensioner housing includes an upstanding rim arranged about the plunger at the open end of the cylinder for retaining oil expressed from and supplied to the dashpot. However, during hard turns of the automobile the oil tends to spill over the rim and air is permitted to be drawn into the dashpot.
To at least partly overcome this problem, in a more recent known chain tensioner design, a further annular piston is arranged in the open end of the cylinder, sealed about the plunger and cylinder wall by O-rings and biased outwardly to apply a positive pressure to the oil in the chain tensioner. This design was intended to avoid the aforementioned spillage problem by providing a completely sealed, pressurized chain tensioner.
During periods of time when the engine is not operating, it is possible to collapse the dashpot, that is, urge the chain tensioner plunger into the cylinder, by the force of a compressed valve spring. This occurs because the valve stem of the valve associated with the compressed valve spring bears with sufficiently great force upon a cam on the camshaft that the camshaft is caused to rotate. This rotation of the camshaft is transmitted by the timing chain to the plunger of the chain tensioner with a force great enough to overcome the force of the coil spring arranged behind the piston. Because the force of the compressed valve spring is relatively constant, large and long duration under these circumstance, the dashpot valve will bypass oil from behind the piston and thereby cause collapse of the plunger into the cylinder. Upon start-up, this long duration force is relieved and the dashpot piston will be urged outwardly to its normal operating position by the coil spring and oil will be sucked back into the dashpot chamber. If insufficient oil is retained in the upstanding rim of the first of the prior art chain tensioners described above, air will be drawn into the dashpot.
The seals of the annular piston of the modified prior art chain tensioner described above are subjected to damage, for example, by foreign matter in the oil, by damage caused during normal wear, by heat and age hardening of the seals and the like. If these seals are or become damaged, it is possible and, indeed, likely that oil will leak past the seals so that air will be drawn into the chain tensioner cylinder, especially during the start-up conditions described above. When this occurs, the chain tensioner is essentially inoperative and the aforementioned failures can result.